The Sea Shore

by William S. Furneaux

Although the seven fins above mentioned differ considerably in general form, some being fanlike, while others form fringe-like expansions, yet they generally agree in that they consist of bony or cartilaginous rays, between which is a soft membrane. The rays, however, vary much in character, being sometimes developed into very hard and sharp spines, and sometimes quite soft and flexible. The fins also differ in function, as will be seen when we observe the movements of a fish as it swims. It will then be noticed that the caudal fin, which is spread in the vertical plane and moved sharply from side to side by the powerful muscles of the tail, is the chief propelling organ, while the others are concerned principally in maintaining the balance of the body. This latter point becomes much more evident when we observe the movements of a fish in which one or more of the fins have been injured or lost, as we shall see presently.

It is true that the pectoral fins are sometimes used to propel, but forward movement is brought about almost entirely by the caudal fin, which acts much in the same way as the blade of the propelling ‘screw’ of a steam-vessel, the pectorals being used at the same time for steering. Occasionally, too, the latter are both spread out at right angles to the body when the fish desires to stop suddenly, and are even employed at times in swimming backwards.

When a fish wants to turn to one side, it will be seen to give the tail a sharp motion to the opposite side. The pectoral of the latter side is also brought into play, while the other is kept close against the body.

If the pectoral or pelvic fin of one side is injured, the body of the fish will incline to the opposite side; and if all the paired fins are functionless the fish swims with its head inclined downwards. Observations of fishes in which the dorsal or ventral fins are injured will also show that these organs are necessary to maintain a steady motion in the water.

In addition to the above facts, it may be mentioned here that the paired fins are often modified into long finger-like processes that serve as organs of touch, and even as means by which the fish can creep along the bottom. This is notably the case with gurnards and a few of the other fishes that spend their time almost exclusively on the bed of the sea.

Fishes are essentially gill-breathers, the gills being generally fringe-like organs, supported on bony arches (the gill arches), numbering four on each side, the cavity containing them being covered by a gill-cover (operculum) that opens behind. Water is taken in at the mouth, whence it passes into the gill-chamber; and after passing between and around the gills, it escapes under the opercula. The gills themselves are richly supplied with bloodvessels that are distributed close to the surface, and an exchange of gases takes place through their exceedingly thin walls, carbonic acid gas passing from the blood to the surrounding water, and oxygen, held in solution in the water, passing from the water to the blood.

When fishes are in foul water, containing but little oxygen in solution, they rise to the surface in order to make up the deficiency by taking oxygen direct from the air. This, however, is an unnatural proceeding with the majority of fishes; but there are some that are provided with accessory breathing organs specially adapted to the extraction of oxygen direct from the air, and these are so dependent on the supply from this source that they are suffocated if prevented from reaching the surface.

In other fishes, such as the sharks and rays, the gills are of an entirely different character from those described above, for they are pouch-like and five in number on each side, each pouch communicating with the pharynx as well as with the exterior by a slit-like opening.

Before leaving the external characters of fishes we must say a word or two about their forms and colours. As regards the former, it is well known that fishes are well adapted for rapid progression through water, but there are many exceptions to this rule. These exceptions, however, apply principally to those species that have no need to swim rapidly, and a study of their habits will show that their form is just as perfectly adapted to their mode of life. They are often species that live on the bottom, or hide in the crevices and holes of rocks, and examples will be given in our future descriptions.

Variations in colour are even more interesting, especially as they are so commonly connected with the nature of the surroundings and the protection of the animals. In nearly all cases the colour is darker on the upper surface than on the lower, thus making it appear that the influence of light has something to do with the formation of the pigments of the skin, and experiment proves that this is, at least to a certain extent, the case; for when fishes have been kept for some time in an aquarium into which light is admitted through the bottom only, pigment spots have formed in the skin on the lower surface.